The alkali roasting of chrome ore will provide a product that when leached with water yields an aqueous alkaline solution containing alkali metal chromate. This solution can then be reacted with acid to yield the dichromate. Sulfuric is a useful acid and a process employing same has been taught in U.S. Pat. No. 2,612,435. Carbon dioxide is also useful, and a showing of its use has been made in U.S. Pat. No. 2,931,704.
It has not been unusual for the roasting of the ore to introduce chloride ions which contaminate the aqueous solution as sodium chloride. In order to remove this sodium chloride impurity, it has been taught in U.S. Pat. No. 3,454,478 that the major processing steps can be supplemented with a two-compartment electrolytic cell. This cell will be located along side the processing stream, and before the sodium dichromate crystallizer. The cell can be a small bleed stream that is electrolyzed, thereby removing the chloride as chlorine gas at the anode, and the dichromate liquor from the anode compartment of the cell is returned to the main process stream.
In U.S. Pat. No. 2,099,658, it has been disclosed to electrolytically produce chromic acid using a sacrificial anode. The process yields a contaminated product or requires an ostensibly cumbersome and inefficient stepwise procedure to achieve relatively impurity-free acid.
It has also been taught, as in Canadian Pat. No. 739,447, that sodium dichromate can be fed directly to the anode compartment of a two-compartment cell in the process of preparing chromic acid. The efficiency of such operation, however, has not proven satisfactory.